1. Field of the Invention
The present invention relates to an optical fiber cable, and more particularly to an optical fiber cable for air-blown installation between a telephone office and a subscriber network.
2. Description of the Related Art
An optical fiber cable serves as a medium for transmitting optical signals through optical fibers. It includes optical fibers, tensile members, a waterproof member and an outer sheath to protect and improve the physical characteristics of the stranded optical fibers. The optical fiber is a glass wire which is made from material such as silica, mainly from quartz, and is easy to handle and install since its diameter is as small as that of a hair. The optical fiber includes a core for transmitting light therein, a clad for confining the light propagating within the core, and a sheath for surrounding the clad.
Optical fiber has have many advantages over coaxial cable, but is problematic in that optical fiber is susceptible to physical and environmental effects. Accordingly, optical fiber cables, which have superior physical characteristics and installation efficiency in comparison with those of the optical fiber, have become widespread. Specifically, indoor optical fiber cables are generally used as networks for connecting telephone offices as well as means for connecting subscribers.
FIG. 1 shows a prior art optical fiber cable having a central tensile member 110 positioned at the center thereof, a plurality of tubes 120 arranged about the central tensile member 110, an outer sheath 130 binding the tubes 120 and the central tensile member 110. The cable further includes rip codes 140 positioned in an empty space between the tubes 120 and on the inner wall of the outer sheath 130.
The central tensile member 110 provides the optical fiber cable with a tension-resistant force and is positioned at the center thereof. The central tensile member 110 may be made from a material such as an FRP (fiberglass reinforced plastic).
Each of the tubes 120 has a plurality of tight-coated optical fibers 120 stranded therein and a filler 122 such as strands of aramid yarn to occupy the empty space. The filler prevents moisture from penetrating into the tubes 120 and relieves any shock applied to the optical fibers 121.
The outer sheath 130 is positioned at the outermost side of the optical fiber cable and is made of a polymer compound such as a PVC (polyvinyl chloride) or a PE (polyethylene). The outer sheath 130 is formed by an extrusion process.
The rip codes 140 are positioned in the empty space between the tubes 120 or on the inner wall of the outer sheath 130 to easily peel off the optical fiber cable. The rip codes 140 may be made of aramid yarn or glass yarn.
Shortcomings of the above-described optical fiber cable include undesired contraction during a change in external surroundings, such as a sudden temperature drop. Also, tensile force during installation in insufficient, because the cable is limited to a single central tensile member.
In an effort to overcome the above-mentioned problem, an optical fiber cable having a plurality of tensile members has been proposed. FIG. 2 shows an optical fiber cable that includes a ribbon tube 210 mounted at the center thereof, a plurality of tensile members 230 positioned around the ribbon tube 210 at a predetermined spacing, and an outer sheath 240 binding the ribbon tube 210 and the tensile members 230.
Within the ribbon tube 210 is a bundle 220 of ribbon optical fibers surrounded by a filler 211. The bundle 220 of ribbon optical fibers is formed by laminating ribbon optical fibers 211 having multi-cores. The ribbon optical fibers 221 are formed by arranging a plurality of optical fibers 221a in a line, applying an UV curing agent thereon, and curing the agent. The empty space of the ribbon tube 210 is filled with a filler 211, such as a jelly compound, acting as a waterproof substance.
The tensile members 230 are positioned on the outer periphery of the ribbon tube 210 at a predetermined spacing and provide the ribbon tube 210 with a tension-resistant force. The outer sheath 240, positioned at the outermost side of the optical fiber cable, is formed by an extrusion process.
However, the optical fiber cable cannot be easily installed in a crooked pipeline or a curved pipeline due to poor flexibility caused by the tensile members.